Construction of saw devices

ABSTRACT

A silicon or quartz wafer for forming a SAW device is the subject of grinding and lapping operation to form its basic shape. The opposing surfaces, as well as the edges extending therebetween, are the polished to reduce the number and size of defects in the surfaces. Metal is deposited onto one of the opposing surfaces which, in use, will be under compression, to form electronic components thereon, and a multi-metallic coating having an outer layer formed of gold is applied to the other surface to form a solder pad by means of which the wafer may be fastened to a shaft or the like by soldering. Martensitic stainless steel is used as a mount, saddle or housing for the SAW substrate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to improvements in the construction ofquartz and silicon SAW substrates such as SAW (Surface Acoustic Wave)devices, and in particular to improvements in the surface finishing andpackaging of such devices.

2. The Prior Art

The ultimate tensile bending strength of a brittle material depends notonly on its size and stiffness but also on the presence of pre-existingdefects. When a quartz SAW substrate, such as a SAW device, is subjectedto bending, for example simple 3-point bending, the surface on theoutside of the bend is placed in tension whilst the surface on theinside of the bend is placed in compression. Any pre-existing defectwhile exists in the surface under tension will, then, be an area ofweakness and hence likely be the initial source of any failure of thecomponent under bending. The failure strength under bending will,therefore, be limited by the size of the largest pre-existing defect inthe component.

Conventionally, quartz SAW substrates are produced by grinding andlapping operations, which results in a large number of small defects onthe surfaces thereof whose size is characteristic of the grinding andlapping processes. The compressed surface of the component is thenfinished by polishing so as to facilitate deposition of metal thereto toform the various components of the SAW device. Traditionally, however,the tensioned surface has not been so finished for two reasons: firstly,because the extra costs involved in polishing both surfaces of thecomponent was deemed unnecessary and secondly, because the unpolishedsurface was found to suppress reflection of the bulk wave duringoperation of the SAW device, thereby reducing parasitic losses whichresult from those reflections.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a method offinishing SAW substrates by polishing opposing surfaces thereof.

According to another aspect of the invention there is provided ametallization layer on a surface of the SAW substrate which serves as asoldering pad.

The invention further includes SAW substrates manufactured according tothe above noted methods.

According to a further aspect of the invention there is provided AuSnsolder for soldering the SAW substrate to a structural member.

In another aspect of the invention the SAW substrate is bonded to astructural component using a glass frit.

According to a further aspect of the invention a martensitic stainlesssteel mount is coupled to the SAW substrate.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to one aspect of the present invention there is provided amethod of production of SAW substrates, such as quartz or siliconcomponents wherein following grinding and lapping operations, opposingsurfaces of the component are polished so as to reduce the number and ofsize of the defects in the surface.

The present invention further provides a SAW device composed of a quartzSAW substrate having a first surface upon which metal is deposited toform components of the SAW device and which, upon bending of the deviceduring use, will be under compression, and a second surface oppositesaid first which, upon bending of the device in use, will be tensioned,both said first and second surfaces being polished.

The present invention offers the advantage that a very significantincrease in the bending strength of the SAW device is achieved. Furtherimprovements may advantageously be achieved by also polishing the edgesof the SAW device in order to eliminate any stress raisers resultingfrom the cutting of the device from the wafer.

In some applications, components such as SAW devices are attacheddirectly to test apparatus, such as a shaft, rather than being housed ina case or the like which is then suitably fastened in place on the testapparatus. Such components may be glued in place by using conventionaladhesives, but the mechanical properties of the resulting bond have beenfound to reduce the responsiveness and sensitivity of SAW devices.Instead, therefore, it has been found to be advantageous to fasten sucha SAW device by high temperature soldering, which may be achieved byproviding a metallization layer on the bonding surface of the substrateof the device. Soldering has the advantage of greatly improving thetransfer of strain and thermal properties of the transducer and henceimproves the accuracy and sensitivity of a SAW device.

The present invention further teaches the provision of a metallizationlayer on the surface of a component such as a planar quartz component,the metallization layer being formed of a multi-metallic coating havingan outer layer formed of gold, as well as a method of fastening such aplanar quartz component such as a SAW device, to a structural componentsuch as a shaft by means of soldering using AuSn eutectic compositionsolder.

This has the advantage of bonding well to the metallized layer,particularly if a multi-metallic coating is applied to the bondingsurface of the SAW device with the outer coating thereof being gold, andcouples the SAW device particularly effectively to the stress field ofthe structural component which it is intended to measure due to the highstiffness (E approximately 68 GPa), tensile strength (approximately 275MPa) and melting point (approximately 280° C.) of AuSn enabling it toact as a good strain transfer medium.

Unlike conventional polymeric backed foil strain gauges, single crystalquartz is a stiff material (E approximately 80 GPa), and the stresslevels required successfully to transmit strain from a structural memberformed of, for example, steel, to a quartz SAW device are necessarilyhigh. As a result creep will manifest itself at much lower temperaturesif a conventional strain gauge adhesive, such as a conventionalpolymeric strain gauge adhesive, is used. The use of AuSn, in contrast,results in much lower levels of creep and hysteresis at the hightemperatures, which can be up to 125 degrees centigrade, typicallyencountered in automotive applications.

AuSn also has the benefit of high thermal conductivity, therebyminimizing thermally induced strain gradients, and hence furtherimproving accuracy of the device.

Instead of soldering, the SAW substrate may instead be bonded directlyto a structural member using glass frit, such as 80% silver and 20%glass, preferably at a temperature in the range of 400-450° C. In thisway no metallization layer is required.

In other applications, quartz and silicon components such as SAW devicesare housed in or mounted on a separate structure such as a box, a saddleor the like, which separate structure is then fastened to a structuralcomponent or within a test environment The performance (repeatability,linearity, hysteresis and creep) of a sensor incorporating a SAW orsimilar device will, in such cases, then depend on maintaining not onlyall the component parts of the device itself within their elastic rangefor all operating conditions, but also the components of the structurein which the device is enclosed or mounted, such as the lid and base ofa case, in their elastic range during operation.

Conventionally, silicon and quartz devices for electronic applications,are packaged in materials such as austenitic stainless steel, kovar oreven plated mild steel, and these materials work well for applicationswhere the device is essentially decoupled from the environment, sincethey can easily be formed and provide an effective bather againstcorrosion etc. However, these materials do not have a high elastic limitand are likely to give rise to non-linear behavior in applications wherethe device must be coupled to the environment for its operation, such astire pressure sensing applications of SAW devices.

In accordance with a further aspect of the present invention, then,silicon and quartz devices for electronic applications are, instead,packaged in or mounted on martensitic stainless steels, in particularprecipitation hardened martensitic stainless steels. Such materials havethe advantage that they have high elastic limits which promote goodsensor performance while still providing protection against corrosion.17-7PH and 17-4PH stainless steel have been found to provideparticularly effective results.

The various references herein to SAW substrates include but are notlimited to sensors based on a high-Q resonant structure or severalstructures sensitive to physical quantities such as mechanical strain,temperature, moisture etc., for exampled SAW (Surface Acoustic Wave)resonators, STW (Surface Transverse Wave) resonators, FBAR thin filmbulk acoustic wave resonators, dielectric resonators etc.

1-16. (canceled)
 17. An apparatus comprising a packaging for a SAW substrate at least partially formed of martensitic stainless steel.
 18. The packaging according to claim 17, wherein said stainless steel is hardened.
 19. The packaging according to claim 17, wherein said stainless steel is 17-7PH or 17-4PH stainless steel.
 20. The packaging according to claim 17, wherein the packaging includes a first section on which, in use, the SAW substrate is carried, which is formed of said martensitic stainless steel.
 21. The packaging according to claim 17, having a chamber formed therein for receiving a SAW device.
 22. A device for protecting a substrate in a non-environmentally-isolating package, comprising: a sensor including a SAW substrate; and a martensitic stainless steel mount coupled to said SAW substrate, wherein said martensitic stainless steel mount possesses a high elastic range so that said sensor is able to monitor the environment through said mount.
 23. The device according to claim 22, wherein said martensitic stainless steel mount includes precipitation hardened martensitic stainless steel.
 24. The device according to claim 22, wherein the matensitic stainless steel is selected from the group consisting of 17-7PH stainless steel and 17-4PH stainless steel.
 25. The device according to claim 22, wherein said martensitic stainless steel mount comprises a saddle for fastening the sensor to a structural component.
 26. The device according to claim 22, wherein said martensitic stainless steel mount comprises a housing that encloses said sensor. 